Siraitia grosvenorii is a plant of the Cucurbitaceae family, native to Zhuang Autonomous Region of Guangxi, China. Fruits of Siraitia grosvenorii have a very sweet taste, and extracts from the fruits are used as natural sweeteners. Moreover, dried fruits of Siraitia grosvenorii are used as Chinese herbal medicines.
Fruits of Siraitia grosvenorii are known to contain mogrosides as sweet components. Mogrosides are glycosides wherein glucose is linked to the aglycone, mogrol. Mogrosides are classified into various types according to the position of linkage of glucose or the number of glucose units. Mogroside V, mogroside IV, siamenoside I, and 11-oxomogroside are contained in fruits of Siraitia grosvenorii. Other mogrosides are also known, such as mogroside I, mogroside IVA, mogroside III, mogroside IIIA1, mogroside IIIA2, mogroside IIIE, mogroside IIA, mogroside IIA1, mogroside IIA2, mogroside IIB, mogroside IIE, mogroside IA1, and mogroside IE1.
These mogrosides have been shown to have a variety of bioactivities. For example, mogroside V has been reported to have the function of regulating insulin secretion in vitro (Non Patent Literature 1: Yao Xue Xue Bao, 2009, 44, 1252-1257). Mogroside III has also been reported to show maltase inhibitory activity in the intestinal tract, and to suppress a rise in blood glucose level (Non Patent Literature 2: J. Agric. Food Chem. 2005, 53, 2941-2946).
While such mogrosides can be prepared by purification of extracts of fruits of Siraitia grosvenorii, several other methods are also known for the preparation of mogrosides. For example, a method for preparing various mogrosides by glycosylation of mogrol with a UDP-glucosyltransferase has been disclosed (Patent Literature 1: WO 2013/076577). Furthermore, a method for preparing various mogrol glycosides from Siraitia grosvenorii extracts with an Aspergillus niger-derived pectinase has been disclosed (Patent Literature 1: WO 2013/076577).
It has been disclosed that yeast (Saccharomyces cerevisiae) has an activity to convert mogroside V into mogroside IIIE, and the yeast gene responsible for this activity is EXG1 (GH family 5, β-1,3-glucanase) (Non Patent Literature 3: J. Agric. Food Chem, 2013, 61, 7127-7134).
Furthermore, koji mold is known as an organism that secretes various hydrolases, and its genomic information is known. Although about 40 genes exist that are considered to encode β-glucosidase-like proteins, there is little information regarding the substrate specificity of the protein encoded by each of the genes. It has been reported that the β-glucosidase of the glycoside hydrolase (GH) family 3 encoded by the AO090009000356 gene of koji mold hydrolyzes disaccharides with a β-glucoside bond (Non Patent Literature 4: Biosci. Biotech. Biochem. 1764 972-978 (2006)). Specifically, its specificity for hydrolysis is the highest for laminaribiose with a β-1,3 linkage, followed by β-gentiobiose with a β-1,6 linkage, cellobiose with a β-1,4 linkage, and sophorose with a β-1,2 linkage.